A. Technical Field of the Invention
The present invention relates to telemetry-based sensing systems that measure physical, chemical and biological parameters. More specifically, these sensing systems comprise a small, modular, low-power implantable biotelemetry system capable of continuously sensing physiological characteristics using implantable transmitters, a receiver, and a data acquisition system to analyze and record the transmitted signal over several months. The preferred embodiment is a preterm labor and fetal monitoring system.
B. Description of the Prior Art
A number of fetuses suffer from diaphragmatic hernia, a condition in which a hole in the diaphragm allows internal organs to shift from the abdominal cavity into the chest cavity. The lungs have insufficient space to develop and about 60 percent of children born with this condition die. An open hysterotomy (similar to a Cesarean section) surgery to access the fetus is used to correct this anomaly. Recently, pediatric surgeons at the UC San Francisco Fetal Treatment Center (FTC) have developed a new surgical procedure to treat diaphragmatic hernia in fetuses through a minimally invasive procedure using endoscopic techniques. With this approach, in utero surgical procedures are conducted through a number of small openings in the uterus, the largest being about 10 mm. This approach has been developed to minimize postoperative preterm labor, a problem encountered in all patients who undergo fetal surgery. Accurate monitoring of uterine contractions in the postoperative period is critical to develop medications that can inhibit the progression of preterm labor.
Among other applications, the inventive concepts disclosed herein can be applied to the general problem of preterm births, which currently account for over seven percent of all live births and with associated costs running into the billions of dollars each year. Various attempts have been made to monitor fetal characteristics by telemetry. U.S. Pat. Nos. 5,431,171 and 5,538,005 issued to Harrison et al. disclose a remote sensing unit with a transceiver that outputs the sampled fetal temperature and electrocardiogram signals to an antenna external to the patient""s body. However, the packaging technique of the prior art biotelemetry units resulted in a unit too large to be safely and conveniently implanted into a patient and with too short an operating time for what is necessary to monitor recovery. Further, these prior art units are of modular in that they operate on only one frequency and they do not measure electrochemical or biological parameters.
In the field of laparoscopic surgery generally, there is a need for implantable biotelemetry devices that continuously measure physical, chemical and biological parameters in vivo for an extended period of monitoring. A pill-sized biotelemetry transmitter small enough to be introduced into the patient through a 10 mm trocar is needed. More specifically, such a device could provide pediatric surgeons with otherwise unattainable information on intra-uterine pressure and temperature as well as fetal heart rate and tissue pH, parameters critical to the successful monitoring and evaluation of postoperative preterm labor and fetal health and well-being.
The present invention provides a transmitter for measuring at least one physiological characteristic from a position inside a subject and transmitting physiological data to a remote location. The transmitter includes a first sensor for a first physiological characteristic and a second sensor for a second physiological characteristic. The first sensor drives a gated integrator that integrates to a fixed threshold voltage. The magnitude of the first physiological characteristic determines the integration time of the gated integrator. A comparator detects when the threshold voltage is exceeded and converts the first physiological characteristic into a pulse of varying width (nominal width 5-15 ms). An edge detector converts each edge of the pulse into a short pulse (nominal width hundreds of microseconds). The short pulses are used to enable an RF oscillator to transmit the physiological data to the remote location. This process is repeated every 100 to 500 ms depending on the value sensed by the second physiological sensor. The one-shot is triggered on the falling edge of the comparator output. The first physiological characteristic is encoded by the time between the two short pulses and the second physiological characteristic is encoded by the repetition rate of the short pulse pairs.
The telemetry system of the present invention for measuring at least one physiological characteristic transmits the individual characteristics to a remote location. The system includes a transmitter for transmitting physiological data as a pulse interval modulated signal. A receiver is configured for receiving and converting the pulse interval modulated signal into a digital pulse stream. The receiver decodes the digital pulse stream into voltages proportional to the individual characteristics of the physiological data. A data acquisition card is configured to digitize the physiological data. A signal processing system is included for the display and storage of the individual characteristics of the physiological data over time.
Advantages of the present invention include a biotelemetric system that utilizes a small, modular, low-power/long-life, pill-shaped implantable biotelemeter capable of continuously sensing physiological characteristics over a period of up to nine months or slightly more. Further, the present invention discloses the integration of ion-selective microelectrode sensors into a biotelemetric system.
The present invention also discloses a method of measuring at least one physiological characteristic from a position inside a subject and transmitting the individual characteristics as physiological data to a remote location. The method includes the steps of: sensing and transmitting physiological data in a pulse modulated signal to the remote location; receiving and converting the pulse interval modulated signal into a digital pulse stream into voltages proportional to the individual characteristics of the physiological data; digitizing the physiological data; and displaying and storing of the individual characteristics of the physiological data over time.